1. Field of the Invention
This invention relates to fiber optic couplers and, more particularly, to a method for monitoring optical fiber lapping and polishing.
2. Description of the Prior Art
Optical energy transmission between parallel, optical waveguides is made possible by the interaction of evanescent electromagnetic fields. This interaction has been used efficiently in the last few years to develop optic-fiber couplers.
In optic fibers, electromagnetic energy is largely transmitted along a high refractive index core. Surrounding the optic core is a low refractive index cladding that provides optical insulation and protection. Evanescence or dispersion of transmitted energy occurs from the optic core outwards to the cladding. The amount of energy extending out from the core decays exponentially with radial distance from the core axis.
Bi-directional optic-fiber couplers rely upon evanescent characteristics for transfer of optical energy between waveguides. A curved optic fiber exhibits energy loss because of evanescence. Optical energy escapes outwards from the convex portion of the curved optic fiber along a plane passing through the optic fiber core and containing the curve. The inverse is true of optical energy incident upon the outer curved portion of the fiber. The amount of optical energy lost or gained increases with a loss of the optic fiber cladding. U.S. Pat. No. 4,431,260 issued to Palmer teaches a method of fabricating a bi-directional optic coupler.
The preferred technique for grinding an optical flat on a fiber implements a mechanical lapping procedure, providing mechanical stability, and precision reproducibility. In this technique, an optic fiber is mounted in a groove on a curved form with a radius of curvature typically between 5 cm and 8 cm. Epoxy adhesive holds the fiber on the form.
Several optic fiber forms are coupled to a lapping fixture which is mounted on a lapping and polishing machine. An optical flat is lapped on the convex portion of each of the optic fibers, nearly exposing a region of optic-fiber core. Two such fibers are placed in face-to-face contact for the required proximity of the evanescent field interactions. Matching index oil is absorbed between the flats by capillary attraction, providing optical homogeneity. The splitting ratio is fine tuned by aligning the fiber surfaces.
Because of the spatial relationship of the fiber cores, there is a high degree of evanescent coupling between the transmitting fiber and the receiving fiber. Maximum transmission depends on the optic fiber thickness, radius of curvature, and the area of the optic flat. Usually, optic-fiber flats are lapped into the cladding to within a few microns of the core.
The depth of the optic-flat surface is determined by microscopically measuring the cross-sectional area of the flat. This process is tedious, for it requires that the optic fibers be removed from the lapping fixture, washed, measured, and replaced on the fixture. This process may be repeated several times before the desired depth is achieved, and usually takes many hours.
It is the purpose of my invention to increase optic flat production by reducing the time of the lapping process.